Atheroclerosis is a major cause of morbidity and mortality in insulin-resistant states such as metabolic syndrome and obesity. Defective regulation of macrophage lipid metabolism plays a central role in the development of vascular lesions associated with these dyslipidemic states. During atherogenesis, macrophages respond to the challenge of lipid excess through enzymatic production of oxysterols and activation of feed-forward nuclear receptor pathways that induce transcriptional programs involved in lipid uptake and efflux, and negatively regulate inflammatory responses. In recent studies, we have shown that the products of the Niemann-Pick type C (NPC) disease genes, NPC1 and NPC2, are required for delivery of lipoprotein-derived cholesterol to sites of intracellular oxysterol synthesis. In this proposal, we will test the hypothesis that macrophage-derived oxysterols play a critical role in the regulation of lipid homeostasis in the vascular wall. We propose that impaired synthesis of side-chain oxygenated sterols will reduce cholesterol efflux and increase cholesterol accumulation in macrophages, and, in concert with increased levels of oxidized cholesterol, promote cytotoxicity and lesion formation. Production of specific macrophage-derived cholesterol metabolites additionally may play a critical role in governing plasma lipoprotein levels through regulation of hepatic lipoprotein synthesis and/or clearance of plasma lipoproteins. This hypothesis will be tested by the following specific aims: 1) To determine whether macrophage-specific NPC1 and NPC2 loss of function alters macrophage oxysterol synthesis, thereby disrupting lipid homeostasis and promoting atherosclerosis in a murine model, 2) To determine whether macrophage-derived cholesterol metabolites affect regulation of plasma lipoprotein levels in chimeric mice with macrophage-specific NPC1 and NPC2 loss of function, and 3) To assess the relationship between plasma oxysterol levels and coronary heart disease and determine if oxysterol levels change following weight loss in humans with the metabolic syndrome. These studies will contribute to our understanding of the role of macrophage-derived cholesterol metabolites in atherogenesis and regulation of lipoprotein metabolism. This project has the potential to transform the care of people with the metabolic syndrome by establishing these metabolites as novel biomarkers for detection of subclinical atherosclerotic disease.